The alteration of the shape and color of keratinic fibers, particularly of hairs, represents an important area of modern cosmetics. In this way, the appearance of the hairs can be adapted both to current fashion trends and to the individual desires of the individual. A person skilled in the art is familiar with various coloring systems for altering hair color depending on the coloring requirement. For permanent, intense colorations with good fastness characteristics and good gray coverage, oxidation dyeing agents are usually used. Such dyeing agents usually include oxidation dye precursors—so-called developer components and coupler components—that form the actual dyes under the influence of oxidizing agents such as hydrogen peroxide, for example. Oxidation dyeing agents are characterized by outstanding, long-lasting coloring results but are also associated with a certain degree of damage to the hair.
If the user would like to reduce hair damage or alter the hair color only temporarily, dyeing agents can be used that are based on direct dyes. In these, dyes that are already completely formed diffuse from the dyeing agent into the hair fiber. In comparison to the oxidative hair coloration, colorations obtained using direct dyes are less colorfast and wash out more quickly. Gray coverage effects that can be achieved using direct dyes also generally have room for improvement. However, the lesser damaging of the hair associated with coloration with direct dyes is advantageous.
Those skilled in the art have long known that the pH value of a hair dye can have a massive influence on the intensity of and the nuancing achieved with the coloration.
For one, the extent of the swelling of the hair depends on the pH value of the colorant. As a rule, the following applies: the more alkaline the agent applied to the hair, the greater the swelling of the hair. More pronounced swelling of the hair, in turn, promotes the diffusion of the colorants into the hair fibers. When set to higher pH values, dyes—particularly those that include neutral direct dyes (nitro dyes, for example) or cationic dyes (such as azo dyes with quaternary ammonium group, for example)—generally produce a more intense color.
Conversely, when performing coloration using acid dyes—i.e., dyes having at least one anionic charge in the form of a carboxylate or sulfonate group—an acidic pH value must usually be set in order to ensure sufficient color uptake. The color that can be achieved with acid dyes is thus generally intensified as a result of the lowering of the pH value.
The user would like to color his hair in precisely the shade that is also indicated on the packaging of the colorant. In order to achieve predictable coloration results with reproducible color intensity, is therefore essential to precisely maintain the exact pH value in the colorant.
For another, the absorption spectrum of a colorant, and hence its color, can also be influenced by the pH value. This is true of all colorants that can be protonated or deprotonated according to the principle of acid-base indicators and whose chromophoric system is influenced by corresponding protonation and deprotonation. Certain acid dyes in particular can react to slight changes in the pH value with a shift in their absorption spectrum. To achieve hair coloration with reproducible nuancing, the targeted and precise setting of the pH value is therefore also of central importance.
In principle, a person skilled in the art knows that the pH value can be adjusted in a hair dye using buffer systems. In U.S. Pat. No. 6,248,314, for example, the pH value of colorants that include not only various acid dyes but also alkylene carbonates as penetration enhancers is set using a buffer system consisting of lactic acid and caustic soda solution.
In GB 2259717 as well, colorants based on acid dyes and organic solvents are described whose pH value is buffered to a range of 3 through the use of citric acid and caustic soda solution.
In view of their long-term stability, however, none of these buffer systems known for colorants can be regarded as being optimal. Either to increase the solubility of the dyes or as penetration enhancers, most colorants based on direct dyes additionally include significant quantities of solvents (benzyl alcohols, propylene carbonates, alcohols, phenoxyethanol, etc.). It has been found that all buffer systems that are based on the use of organic acids (such as citric acid, etc.) can tend to exhibit increased crystallization particularly in these solvent-containing formulations. If a substantial portion of the buffer system crystallizes out, then it is no longer available for the acid-base equilibrium on which the buffering effect is based, and the buffer capacity can decrease.
It is therefore desirable to provide colorants based on direct dyes that can be set precisely and reproducibly to the desired pH value without encountering the drawbacks described above. The pH value of the colorants should not change, above all even over the course of extended storage.
Moreover, the pH value of the colorant should be unforeseeably influenced neither by the use of raw materials of different qualities or specifications not by the use of water of different water hardnesses.
Finally, the colorants provided in this manner should be suitable for all types of packaging, particularly including for carrier systems that include high levels of solvent, and for use in the form of an aerosol.
Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with this background of the invention.